Carantoc said...

Is it just a coincidence

we don't like coincidences on this site. It's a conspiracy if things appear to align

If you took your kite out at Lennox Head the other week you may have set a new record

ozpricey said...

Its to do with the viscosity of water.

I will use an analogy:
If you try to cook some noodles at the top of Mt Everest you'll find you'll have to cook them longer as the water will boil at a temperature lower than 100*C lets say 80. If you try to cook them at the bottom of a deep mine they'll boil at above 100*C, say 110. Pressure and temperature are directly proportional. Raise one, so will the other.

As you approach the mythical 50kn the fin is splitting the water at such a rate that the low pressure side of the foil is low enough to boil the water at its ambient temp. The formation of air bubbles results in cavitation. Which is not good for speed records. Foils are getting very efficient, but they still require a high and low pressure side which will ultimately result in the same outcome given a high enough velocity.

Obviously water temperature, composition and other factors play a role. But I am led to believe that this limit, enforced by the physical properties of water itself is somewhere around 50kn.

I think kiters have the outright record at the moment. The fact that they can skim across the water rather than sail through it may help their cause. Which is also why they have been classed differently and do not hold the 'sailing' related speed records. ha.ha.

That's simple then, need to get rid of the fin

choco said...
That's simple then, need to get rid of the fin

LOL!

Carantoc said...

But surely a boat hull, a windsurf board and a kiteboard have very different sizes, weights and different fin size ratios, hull lengths etc. but they still have very similar speed records.

Perhaps there is a golden formula for these ratios and the absolute size doesn't matter as much as the ratio, and everybody has found that ratio ?

The land speed record is very different at 110 knots. I guess this could imply what you said and that what stopping water records is not sail design but hull design.

So would it be possible (and valid) to put a hull in a channel of water, accelerate the water flow and measure the force required to hold the hull in the same place ? Would this replicate sailing and would a graph of the force vs water flow therefore be exponential tending toward somewhere around 50 knots ?

But - would this imply it is impossible to accelerate a finned planning hull over the maximum graph speed, 'cause I would have thought that a great big engine could push a finned windsurf hull greater than 50 knots, or is that just a ignoramouses assumption ?

Yeah your right I hadn't thought of that. It must also include the power required to get it to the 'terminal speed' as well. It must be a ratio to do with shear force and viscosity, which makes sense as the low pressure wouldn't be all that low unless it was forced to be, ie in a fin shearing through the water.

This did a 61kn peak
www.hydroptere.com/

I guess if you can keep the low pressure to just under the boiling point you're sweet. Then you just have to worry about the boat taking to the sky

Carantoc said...

So would it be possible (and valid) to put a hull in a channel of water, accelerate the water flow and measure the force required to hold the hull in the same place?

Yep, that's called a towing tank. But the water is stationary and the boat is towed down the tank. Using the principle of Froude-scaling, this can be done with smaller models at lower speeds ie 10m/s in a 100m long tank with a 1m long model.

Carantoc said...

Would this replicate sailing and would a graph of the force vs water flow therefore be exponential tending toward somewhere around 50 knots ?

Yes this would replicate the water flow of full-scale sailing. At very high speeds, the drag is made up of the following components: frictional (water sliding over hull), spray-making and induced drag (a drag that occurs because you are creating lift). As speed increases, the resistance will increase roughly as a square ie R~V^2. So not quite exponential, but getting steep. There is no terminal speed, simply a speed where cavitation becomes a problem, this is around 50 knots in seawater at sea level.

Boats, windsurfers and kiteboarders all have weight that is must be supported by the water. At 50 knots this is predominantly done by dynamic (planing) forces as opposed to static (buoyancy) forces. Creating this lift comes with a penalty in the form of drag because some part of the vessel is moving thru the water. Hovercraft avoid this using a cushion of air to support their weight.

Without some form of centreboard/keel/fin/board edge, a vessel will not move across the wind, it will be pushed directly downwind. It is important to move across the wind to generate apparent wind. So we also need a fin in the water, and this is vulnerable to cavitation.

Laird's foil boards probably use a typical wing section which is the most efficient lifting surface for speeds below cavitation inception. Check out the sailing Moths that use hydrofoils, its exactly the same concept. They minimise drag by getting the hull out of the water. So the only drag is from the foil and the struts/rudder/centreboard.

They are good up till cavitation, then a different type of section is required. Supercavitating foils look like a thin isosceles triangle, very sharp at the nose with a thick square cut tail.

ozpricey said...

I've read a little on supercavitating torpedos its pretty interesting.

whatthe how does the French boat L'Hydroptere get to 61kn does it have those cavitating fins?

That's an easy one... that thing has so much power that it is pushing through the extra drag caused by cavitation. Cavitation isn't a speed limit -> its a point where drag increases dramatically. Windsurfing is power-limited.

From my reading, apparently super-cavitation bubbles aren't stable unless you are exceeding 100kn or thereabouts (or generating your own bubble) - information is limited as it appears most testing has had military purposes.

And all fins cavitate....

Carantoc said...
So - what are the options other than supercavitating fins ?

Just going straight downwind ? Whatever your sailing I guess you would need some balls for this and technically I suppose it wouldn't be classed as sailing, more like horizontal parachuting.

A two stage system, one to get to 50 knot and one to get over 50 knot ?

super-ventilating foils...

can't go straight downwind as the sail wouldn't generate any lift.

2 fins - one that folds up at speed...